SAE Technical Paper 850542 "DDEC Detroit Diesel Electronic Control" describes in detail a method for detecting when a solenoid-operated movable valve element reaches an end position. Injection commences when a closed position is reached. The method is based on studying the flow through, or the voltage across, the solenoid, which flow or voltage undergoes a change when the valve element changes from being in motion to being stationary in an end position. The reaching of the end position is defined as the time when the first-order time derivative of the current (dI/dt) or voltage (dU/dt) passes through zero. The zero crossing is caused by a sudden change in inductance when the valve element becomes stationary in its end position.
U.S. Pat. No. 5,182,517 refers to a more developed variant whereby both the closing and the opening of the control valve, i.e. in this case the commencement and cessation of fuel injection in a combustion engine, are detected by analysing the current through the solenoid. The opening of the control valve, i.e. the interruption of injection, is detected by the fact that a free-wheel circuit opens. Free-wheeling means that the current is led into a circuit with a certain resistance so that the energy which is stored in the solenoid in the form of its magnetic field is reduced. When the current is free-wheeled, this value decreases exponentially and the time when the control valve reaches the fully open position is defined as the time when the value of the current has a local maximum value, i.e. the derivative of the current passes through zero. This method is used for measuring the start and length of an injection cycle for the purpose of controlling the fuel quantity injected and the time when the injection takes place. A disadvantage of the free-wheeling method is that the ill-defined phases when the control valve is set in motion from one end position to the other, particularly when the control valve is opening, are unnecessarily long. This is a disadvantage with regard to exact determination of the fuel quantity injected.
DE 4222650 refers to a variant whereby free-wheeling can be limited on the basis of the engine speed at the time. At low speeds, full free-wheeling is permitted but with successively increasing speed the free-wheeling period is shortened until in the higher speed range it is completely eliminated. This solution is used in order to limit noise from the control valve at low engine speeds, at which the result is a slower control valve movement, with a more rapid control valve movement at higher speeds and consequently more precise determination of the fuel quantity injected. The predominance of other noise sources at these higher speeds then makes it less necessary to limit noise from the control valve.
The documents mentioned above refer to solutions which are applied to fuel injectors for combustion engines whereby the fuel pressure builds up when a solenoid-operated control valve is closed, followed by the injection valve opening when the pressure in the fuel reaches a given level, of the order of a couple of hundred bars. In these arrangements the time when injection takes place is calculated by detecting the time when the control valve closes and adding a time which in principle is constant but depends to some extent on the type of injector and certain conditions such as temperature. This time corresponds to the time during which the fuel pressure against the injection valve builds up. In this case the fuel quantity injected can be detected by also detecting the time when the control valve opens, since the fuel pressure then begins to drop, and the injection valve closes when the pressure drops to a predetermined level.
During the period when the control valve is moving towards the open position, the flow through the control valve is not well defined, thereby unpredictably influencing the closure of the injection valve. The determination of the fuel quantity injected thus becomes relatively uncertain, thereby worsening the possibility of exactly controlling the fuel quantity injected with a view to its corresponding to the desired power output from the engine and at the same time to achieving optimum control of fuel consumption and emissions.
To make the determination of the fuel quantity injected more precise and improve the possibility of controlling the fuel quantity injected during the injection period, it is important that the control valve be adjusted as quickly as possible and that the reaching of both of its end positions can be precisely detected in order to determine the fuel quantity injected.
U.S. Pat. No. 4,856,482 refers to a solution whereby the current through an electromagnetic valve is interrupted for a certain time in order to quickly urge the valve towards the other end position. After a certain time a second lower voltage level is applied to create a measuring current which can then be analysed in order to detect when the end position is reached. However, this solution does involve a relatively complicated circuit solution, since two voltage levels are applied.